1. Field of the Invention
The present invention relates to an O-ring and a clamp-type joint for a vacuum apparatus using the O-ring, which does not cause cracking of the O-ring during use at a high temperature.
2. Background Art
A typical example of a conventional clamp-type joint for vacuum apparatus specified in JIS B8365 (ISO 2861/1) (“Dimensions and shapes of clamp-type joint for vacuum apparatus”, 1988, hereinafter referred to as “JIS standard”) is shown in FIG. 2. The clamp-type joint for vacuum apparatus 100 has a coupling 101 having a pair of parallel flat planes 111 and 111, a center ring 102, of which the outer circumference is a concave groove 112, inserted between the pair of flat planes 111 and 111 of the coupling 101, an O-ring 103 engaged with the concave groove 112 of the center ring 102, and a brim 113 of the coupling 101 which is fastened by clamps (not shown) to seal fluids in the planes 111 and 111 of the coupling 101 and the face of the O-ring 103.
A slight internal pressure (positive pressure) may be applied to the clamp-type joint for vacuum apparatus 100, even in the case in which the sealing object is a vacuum pipe. If an internal pressure is applied, the O-ring of the clamp-type joint expands outwardly, thereby inhibiting stable sealing performance and causing a fluid to leak. In the worst case, the O-ring may come off from the clamp-type joint, thereby inhibiting sealing. In order to obviate this problem and to inhibit outward expansion of the O-ring 103 due to internal pressure, an outer ring 105 with a U-shaped cross-section made of metal is provided around the circumference of the O-ring 103 in a manner to cause the O-ring 103 to be engaged therewith, as shown in FIG. 3. A clamp-type joint for vacuum apparatus 200 having this structure can inhibit outward expansion of the O-ring when internal pressure is applied in a pipe of a vacuum apparatus, thereby ensuring stable sealing performance. In addition, since O-rings used for clamp-type joints have improved heat resistance in recent years, the clamp-type joint used in pipes of vacuum apparatuses are used at a high temperature of 200° C. or more in certain occasions.
However, if the clamp-type joint for vacuum apparatus 200 equipped with an outer ring is used at a high temperature of 200° C. or more, the O-ring 103 may crack, resulting in sealing failure in the worst case.